For many years, it has been customary to employ printed circuit boards ("PCBs") or printed circuit assemblies ("PCAs") as a medium for mechanically holding electronic components together and for providing electrical interconnections among the components. The earliest PCBs were constructed of an insulating planar substrate (such as a glass fiber/resin combination) upon which was deposited a layer of conductive metal. Most typically, the metal layer coated the entire surface of the substrate and was chemically etched to place a pattern of conductors (or "traces") on the surface. Often, metal layers were provided on both the upper and lower surfaces of the substrate to allow conductors to cross one another without making electrical contact. A plurality of mounting holes or "vias" were drilled through the metal layer and substrate. The vias were situated to receive leads from the electronic components (so-called "through-hole" mounting).
To complete assembly of a circuit board, the electronic components were placed on the PCB, either by hand or robotic machine, the leads of the components passing through corresponding vias. Lastly, solder connections were made to ensure reliable electrical contact between the components and the traces. Initially, soldering was performed manually. Subsequently, more efficient machine-soldering techniques employing infrared ovens or solder baths were developed to speed manufacture of circuit boards and to ensure higher solder joint reliability.
Under such machine-soldering techniques, the PCB and its components were heated. Solder, under the influence of flux, was caused to contact the board and flowed by capillary action into the vias, yielding a low resistance solder joint when cooled.
As circuit board technology developed, designers began to create circuit boards comprising many alternating substrate and conductive layer pairs, resulting in sandwiched circuit boards that could accommodate a higher component density. Such boards could accommodate ten or more conductive layers.
Later, surface-mount technology allowed the leads to be soldered to solder pads on the surface of the circuit board, rather than requiring the leads to pass through vias to be soldered therein.
The electronic components themselves underwent changes to accommodate higher density. First, discrete components were combined into integrated circuits ("ICs"). ICs were originally placed in dual in-line packages ("DIPs") consisting of an elongated plastic body encapsulating the IC and a plurality of electrical leads coupled to the IC and arranged in a series extending from the two long edges of the body. The leads could either be through-hole soldered or surface-mounted. Unfortunately, the number of leads that a DIP could accommodate was a function of twice the length of the DIP body edges. Some improvement was made by providing packages having leads extending from all four edges of the body, but, even so, the number of leads was a function of the perimetral length of the body edges.
Next, in an effort to increase lead density further (to address, in particular, the increasing power and density of microprocessors and the stringent space requirements of notebook, subnotebook and personal digital assistant ("PDA") computers), designers developed quad flat packs ("QFPs") comprising a generally square body having leads extending downward from the lower surface of the body. The leads were typically arranged in multiple rows and columns, allowing the QFPs to accommodate more pins than DIPs. However, limitations in socket size and collective lead insertion force began to be problematical.
Currently, designers are focussing on ball grid array ("BGA") packaging wherein leads are dispensed with and replaced with a finely-pitched matrix of conductive contact surfaces on the lower surface of an otherwise conventional body. The circuit board to which a BGA package is to be mounted is provided with a matrix of corresponding surfaces ("pads") upon each of which is deposited a small quantity of solder. To mount the BGA package to the circuit board, the BGA package is temporarily clamped to the board and the board heated (typically by application of infrared energy), causing the solder to melt, fusing the corresponding surfaces together and yielding a strong mechanical and electrical connection when cooled.
BGA packaging is proving to be a powerful ally in the further miniaturization of computers. However, the circuit boards designed to receive the BGAs are lagging in compactness. The problem centers on how to route the electrical conductors from each BGA pad through the circuit board.
In multi-layer boards, electrical signals are routed from layer to layer by metal-coated vias. Accordingly, a via is required for each pad to communicate electrical signals between the pad to a trace on another layer of the board. As will be illustrated, most current circuit board designs employ a matrix of vias that is spatially offset from the matrix of pads and coupled thereto by short traces. Unfortunately, this results in a board layout that is in excess of the footprint of the BGA package, hindering compactness.
Further, it is not possible simply to place a via under each solder pad since, when the BGA package is clamped to the board and the board heated, the solder pads drain into the empty via below, rather than spreading out between the pad and the BGA surface. This results in unacceptably poor and unreliable solder joints.
To overcome this disadvantage, designers developed a "blind via" approach wherein precision via-drilling equipment is employed to drill the vias from underneath the board almost, but not entirely, through the full lateral thickness of the board. Instead, the via is stopped when it reaches a topmost conductive layer that has been divided into pads. Solder is placed on the topmost pad layer. When the BGA package is clamped on and the solder heated, the topmost pad layer prevents the solder from draining into the via.
However, there are two significant disadvantages with this process. First, the precision (in all three axes) that is required of the drilling operation and of the board thickness inevitably raises manufacturing cost and rejection rates. Second, since such vias are plated with a conductive coating on an inner surface thereof, the fact that the via does not pass entirely through the board presents a challenge to conventional via plating techniques, often resulting in inadequately plated vias and high resistance or nonexistent interlayer connections.
Accordingly, what is needed in the art is a less expensive and more reliable method of providing a fine pitch BGA pad matrix on a circuit board.